Before you buy a used BMW Serie 3 (E46).

Look out for: Loss of power, smoke or knocking, often with no prior warning: one of the flaps comes loose and drops into the cylinder.

Ask them to check: Check whether they have already been removed or replaced with blanking plugs and inspect the intake manifold.

Look out for: Usually none until it fails. A common early warning: oil seeping around the flap spindles on top of the engine. If a flap or its screw comes loose and enters a cylinder: sudden heavy knocking, loss of power, smoke and possible turbo damage as well.

Ask them to check: At a garage: confirm whether the manifold has swirl flaps (some versions do not), look for oil around the spindles and play in the flaps, and check the history for earlier replacement or removal. With any new metallic knocking, stop the engine and do not restart it.

Look out for: Little prior warning; once wear is advanced, a metallic rod knock that rises with revs. On the S65 and S85, also jerking and limp mode from failing throttle actuators.

Ask them to check: Oil analysis and bearing replacement history. Indicative preventive replacement: S54 around 130,000–160,000 km, S65 around 100,000–130,000 km and S85 around 80,000–110,000 km.

Look out for: Coolant loss without an obvious puddle, sweet smell, low-level warning, steam in the engine bay, heater stops warming and the temperature needle rises. Expansion tank failure is usually sudden; the water pump can fail with no visible leak.

Ask them to check: With the engine cold, check the level and look for cracks or dried coolant on the tank, radiator neck and thermostat housing. At a garage: pressure-test the system and inspect the pump. If the temperature rises, stop somewhere safe: a single overheat can warp the aluminium head. Never open the system hot.

Look out for: High oil consumption, bluish smoke, rough idle, whistling from a vacuum leak, engine light with lean-mixture codes and very strong suction when trying to remove the oil filler cap at idle.

Ask them to check: At a garage: smoke-test the intake to find leaks (CCV hoses, intake boots, DISA valve seal, valve cover gasket) and check whether the dipstick tube return is blocked. A cold-climate version of the system exists for cold regions.

Look out for: Loss of torque and pull below about 3,000 rpm, louder or unsteady idle, the engine bogs when the air conditioning is switched on and sometimes VANOS fault codes. On 1998–2000 M52TU engines, jolts or stalling on cold starts.

Ask them to check: Read the engine codes and, at a garage, compare actual and requested camshaft positions in live data. Rule out vacuum leaks and the CCV first, as they cause similar symptoms. On M52TU engines with cold jolts, the kit maker describes a test with the intake VANOS solenoid unplugged.

Look out for: On cars built up to March 2000, the rear axle mounting can tear out of the body; worn wishbone bushes (noises and a nervous wheel under braking) and broken rear springs; hairline cracks in the expansion tank; oil pump on the six-cylinders; camshaft sensors, turbos and injection pumps; swirl flaps dropping into the cylinder on diesels; weak handbrake and porous brake pipes.

Ask them to check: On cars up to 2000, inspect the rear axle mounting area on a lift; check airbag campaigns by VIN.